Ant..
sorry..but you have probably killed the tree by keeping it indoors all winter..
Bonsai made from native trees MUST be outdoors all winter..
they need to go dormant in the winter just like any "normal" tree outdoors..
they ARE "normal trees outdoors"!
their seasonal needs dont change just because they are put in a pot..
Bonsai must be grown and kept outdoors 24/7/365..they should never be brought indoors...ever..
it might survive..but the odds are slim.
it will probably run out of steam and die in the spring because it hasnt had a winter rest..
sorry..but you totally did the wrong thing by bringing it indoors..
if you are worried about drainage, many people take their bonsai out of their pots for the winter and bury the root ball in the ground, then heavily mulch...then re-pot in the spring.
this is the exact same thing as trying to grow VFTs and Sarracenia indoors on windowsills all winter..
Maple trees and VFTs *must* have a winter dormancy..its not an option, its a necessity.
Bonsai have always been grown outdoors! for centuries!
its how its done..
the only exception are "tropical" or" indoor bonsai"..made from tropical plants like Ficus trees..
but all Bonsai from native trees are always kept outdoors every day of the year..even, (and especially) in the winter..
Scot